This invention relates to tunable acousto-optical filters.
Acousto-optic (AO) devices are described in textbooks, such as J. Xu and R. Stroud, Acousto-Optice Devices: Principles Design and Applications, Wiley, N.Y., 1992. The AO interaction is a parametric three-wave mixing process where an incident optical beam and acoustic beam interact to generate a second optical beam. The second optical beam generated by an AO interaction is referred to as the first order or diffracted beam. The portion of the incident optical beam that is not converted to the first order beam is referred to as the zeroth order or undiffracted beam.
The efficiency of an AO interaction (i.e. the amount of optical power transferred from the zeroth order beam to the first order beam) is partly determined by energy and momentum conservation considerations, usually referred to collectively as xe2x80x9cphasematchingxe2x80x9d. Energy conservation requires the following relation to hold:
fd=fixc2x1fa,xe2x80x83xe2x80x83(1)
where fi is the frequency of the incident optical beam, fa is the acoustic frequency, and fd is the frequency of the diffracted optical beam. Thus the frequency of the diffracted beam is shifted up or down relative to the incident optical frequency by an amount equal to the acoustic frequency. If momentum conservation is exactly satisfied, then the following relation holds:
kd=kixc2x1ka,xe2x80x83xe2x80x83(2)
where ki, kd and ka are the incident optical, diffracted optical and acoustic wave vectors respectively. The determination of the sign in Eqs. (1) and (2) is discussed later.
Geometrically, this situation can be represented in a phasematching diagram as in FIG. 1A, where the three vectors form a closed triangle. In this case, phasematching is achieved, and the efficiency of the AO interaction is maximized. If Eq. (2) is not exactly satisfied, the efficiency of the AO interaction is reduced, and this reduction in efficiency generally increases as the departure from exact phasematching increases.
The spectral filtering properties of an AO device are mainly determined by phasematching. Consider an AO device where phasematching is exactly satisfied for an incident optical wavelength xcexc, as represented in FIG. 1A. If the incident optical wavelength xcex is reduced, the phasematching diagram is as represented in FIG. 1B. In FIG. 1B, the vector length |ki| is greater than in FIG. 1A, but lies in the same direction. The acoustic wave vector ka has the same magnitude and direction in FIGS. 1A and 1B, because the acoustic frequency is held constant, and the acoustic direction is determined by device geometry. The length of the vector kd increases, due to the energy conservation requirement of Eq. (1). Because the acoustic frequency shift of the diffracted optical beam is a negligible fraction of a typical optical frequency, the vectors ki and kd increase in length approximately proportionally. Given the geometrical constraints outlined above, it is clear that the phasematching triangle cannot be exactly closed for xcex less than xcexc, no matter what the direction of kd is, as indicated in FIG. 1B. Furthermore, as |xcexxe2x88x92xcexc| increases, the departure from the exact phasematching condition of FIG. 2A increases.
These considerations indicate that an AO device acts as a bandpass filter in first order transmission, because only wavelengths that are sufficiently near the center wavelength xcexc are efficiently diffracted from the zeroth order beam to the first order beam. An AO device in zeroth order transmission acts as the corresponding notch filter, as indicated in FIG. 2, where transmission is generally high except for wavelengths near xcexc which are efficiently converted to the first order beam. These optical filters are tunable: changing the acoustic frequency will change the length of the acoustic wave vector ka, and thus change the center wavelength xcexc.
As indicated above, there is a sign ambiguity in Eqs. (1) and (2), where either the sum or the difference of the optical and acoustic wave vectors is indicated. In practice, one of these two alternatives is typically much more nearly phasematched than the other, and is therefore the only relevant possibility. This determination of the relevant interaction fixes the signs in Eqs. (1) and (2), because the signs are necessarily the same in the two equations. For example, FIGS. 1A and 1B correspond to use of a + sign in Eqs. (1) and (2), because it is clear that the difference between kd and ki+ka is much smaller than the difference between kd and kixe2x88x92ka, even if the direction of kd is chosen in both cases to minimize these differences.
In order to realize a first order bandpass filter, or a zeroth order notch filter, with an AO device, it is necessary to suppress the undesired beam. Although the zeroth and first order beams are always distinguishable in principle, due to the frequency shift of Eq. (1), it is more practical to suppress the undesired beam based on other possible differences between the two beams, such as having different states of polarization. For example, S. E. Harris and R. W. Wallace (Jour. Opt. Soc. Amer., vol. 59 (1969) pp. 744-747) disclose a tunable bandpass filter where a separate polarizer is used to suppress the zeroth order beam and transmit the first order beam. U.S. Pat. No. 3,644,015, issued to Hearn, discloses a tunable notch filter where a separate polarizer, spaced apart from the AO converter, is used to suppress the first order beam and transmit the zeroth order beam.
In a co-pending application, U.S. Ser. No. 10/086,283, the use of an AO device operating in zeroth order transmission as a laser tuning element is taught. For such applications, it is advantageous to suppress the first order beam within the AO device itself, to reduce the number of extraneous beams in the laser cavity.
What is needed is an AO filter assembly having no first order beam emission and suitable for use as a laser tuning element. The assembly should provide an output coupler that suppresses the diffracted beam. Preferably, this approach should permit use of collinear and/or non-collinear optical beams. Therefore, different methods of beam discrimination should be provided.
These needs are met by the invention, which provides several methods, which are individually applicable to collinear optical beams and/or to non-collinear optical beams, for receiving and coupling out an undiffracted beam from an AO filter assembly and for suppressing a diffracted beam component so that the diffracted beam does not leave the crystal. Tunability is provided through variation of acoustic frequency.
In each approach, an optical beam and an acoustic beam are introduced into an acoustically and optically anisotropic crystal, at a selected orientation relative to each other, and allowed to interact to produce an undiffracted light beam and a diffracted light beam. The diffracted beam is received by an output coupler, which can be affixed to or be an integral part of the crystal at a face thereof, and is partly or fully suppressed, and a portion of the undiffracted beam exits from the crystal/coupler.
In a first approach, the diffracted and undiffracted beams are noncollinear, and the output coupler is an aperture having a small diameter and positioned to permit passage of most or all of the undiffracted beam from the crystal and to block substantially all of the diffracted beam. In an alternative approach, the diffracted and undiffracted beams are perpendicularly polarized relative to each other, and the output coupler is a polarization-selective component that permits an undiffracted light beam having a first polarization direction to exit from the crystal and suppresses a diffracted light beam having a second polarization direction that is perpendicular to the first direction. In another alternative approach, the undiffracted and diffracted beams have separate polarization directions, and a crystal surface is oriented to receive the two beams so that (i) the diffracted beam polarization direction is in the plane of incidence, and (ii) the diffracted beam approaches the surface at a Brewster angle for this surface, thereby extinguishing the diffracted beam component that would otherwise be reflected from this surface.
In other alternative approaches: (1) a multilayer band pass coating on an exit face transmits light for the undiffracted beam incidence angle but reflects light for the diffracted beam incidence angle; or (2) the diffracted beam is totally internally reflected and a portion of the undiffracted beam is transmitted at an exit face.